1. Field of the Invention
The present invention relates to a dither method and a printing apparatus adopting the dither method.
2. Description of the Related Art
Conventionally known is an electrophotographic color printer which prints an image on a sheet of paper through the use of toners of four colors of yellow (Y), magenta (M), cyan (C) and black (K) by setting each of dots for one color ON (transferring a toner) or OFF (transferring no toner) and thereafter setting each of dots for another color ON or OFF in accordance with image data. In the case of printing a halftone image with the electrophotographic color printer, the number of dots per unit area (e.g. 1 pixel) is varied. This is known as dithering, which is an area gradation technique of comparing the values representing the tones of colors (gradations) in an image to be printed with a threshold value calculated by a predetermined rule and determining printing/non-printing in accordance with whether the values representing the tones are equal to or greater/less than the threshold value.
There are two types of dithering methods. One is a conditional determination method of determining the threshold values of the pixels which are to form an image in a gradation of colors, in consideration of the tones of colors of their circumferential pixels. The other is an independent determination method of determining the threshold values without taking the tones of colors of the circumferential pixels into consideration. Generally speaking, the independent determination method is easier than the conditional determination method, and the processing according to the independent determination method can be executed at higher speed. In light of this, the independent determination method is often employed to print halftone images with a color printer.
However, the color printer which represents halftones according to the above-described dithering methods, in particular, one known as the independent determination method, has the following problem:
Image data items corresponding to pixels forming an image such as a character or figure to be printed are each expressed in two values representing printing/non-printing (ON/OFF). Due to this, when representing halftones according to the independent determination method, mask patterns of the number corresponding to the number of tones of colors in an image to be printed have to be prepared based on a threshold matrix whose elements are expressed using 0 and 1. Therefore, as the number of gradations to be represented increases, the mask patterns of the number corresponding thereto become necessary, which results in a large-capacity memory being required.
The color printer prints a color image on a sheet of paper by printing monochrome images of the colors Y, M, C and K so as to be superposed on one another. In order to represent halftones with the Y, M, C and K monochrome images, the color printer may use the same mask pattern for Y, M, C and K pixels if those pixels are to be printed having the same gradation. In this case, however, moire can occur if the printer fails to print the images of the individual colors in the appropriate positions on the sheet of paper. In consideration of this, the mask patterns of the number corresponding to the number of gradations to be represented have to be prepared for each of the colors Y, M, C and K.